While the Histogram and Waveform Monitor let us analyse the luminance of an image, the Vectorscope allows us to visualise the colour content. Strictly speaking, a Vectorscope is any display which visualises the relationship between two dissimilar signals as a two-dimensional X-Y graph; an audio phase plot, representing the relationship between the two audio streams in a stereo signal, is also a vector display.
The test image as viewed on a vectorscope. Notice the peaks towards cyan and red created by the reflections
Without going into too much television engineering history, most people are aware that colour images are described by three channels – most obviously red, green and blue, but also Y, U and V, where Y represents brightness, and U and V represent the colour information. In digital systems, these channels are generally referred to as Y, Cr and Cb, where the R and B subscripts represent the fact that the two colour channels are calculated by subtracting the red and blue RGB signals from the Y luminance signal, which is, broadly, an average of R, G and B. The purpose of separating colour and brightness in this way is that the human eye detects luminance with more sharpness than it detects colour, so the two colour channels may be stored and transmitted with lower resolution without affecting perceived sharpness. This is the source of the familiar 4:2:2 and 4:1:1 notation, where colour information is sent at half or one-quarter the resolution of luminance. There are technical differences between the approaches referred to as YUV, YCrCb and the YIQ of broadcast NTSC, but for the sake of this article I'll refer to YUV, as the basic concepts are the same regardless.
The test image with hue rotated +90 degrees
The resulting vectorscope display is literally rotated, though counterintuitively, it rotates anticlockwise for a positive hue angle alteration